Despite our advances in understanding risk factors for atherosclerosis, this disease remains the leading cause of death in western countries. This is in part a result of our current limited understanding of the specific pathophysiological mechanisms underlying the disease.
In humans, it is clear that atherosclerotic lesions develop at sites of pre-existing intimal thickening, or intimal hyperplasia (Schwartz et al. (1995) Circulation Research 77, 445-465; Ikari et al. (1999) Arteriosclerosis, Thrombosis, and Vascular Biology 19, 2036-2040; Schwartz (1999) Circulation Research 77, 445-465; Kiechl S. et al. (1999) Arteriosclerosis, Thrombosis, and Vascular Biology 19, 1484-1490; Nakashima et al. (2002) Virchows Archives 441, 279-288; Tracy (2003) Kluwer Acadmeic Publishers, Dordreclt, The Netherlands.). These pre-atherosclerotic lesions are typically eccentric and often located immediately distal to branch sites in the vasculature. The eccentric nature of these lesions and their location near branch sites both suggest that altered shear forces are a predominant etiological factor in their formation. Furthermore, it is clear that applying such altered shear forces to cultured endothelial cells in vitro, does in fact alter the phenotype of these cells, to a phenotype that would be more likely to promote the development of atherosclerosis (Davies (1995) Physiological Reviews 75, 519-160; Garcia-Cardena et al. (2001) Proceedings of the National Academy of Sciences USA 98, 4478-4485; Traub & Berk (1998) Arterioscler. Thromb. Vasc. Biol. 18, 677-685.).
These pre-atherosclerotic lesions are composed primarily of smooth muscle cells and abundant proteoglycan-rich extracellular matrix with scattered macrophages (Stary et al. (1992) Circulation 85, 391-405). Within these thickened intimal lesions, macrophages (and to a lesser extent smooth muscle cells) form foam cells secondary to ingestion of lipoproteins retained in the intima (Stary et al. (1994) Circulation 89, 2462-2478). Although these early lesions form quite frequently even in infancy, they appear to be readily reversible (Stary (2000) American Journal of Clinical Nutrition 72(Suppl.) 1297S-1306S; Stary (2003) Atlas of atherosclerosis progression and regression, 2nd edition, Parthenon Publishing Group, New York). Thus, it appears that the predominant irreversible (or limitedly reversible) step, committing a vessel to clinically relevant atherosclerosis in humans, is the development of a necrotic/lipid core within this hyperplastic thickened intima.
Currently it is unclear why intimal hyperplasia progresses to atherosclerosis much more readily at some locations than at other locations. Along this line of investigation, much attention has been focused on the proteoglycans composing the extracellular matrix of the thickened intima. Proteoglycans have been proposed to play a direct role in atherosclerosis both by binding and retaining lipoproteins in the vessel wall and by regulating vascular cell growth (Berenson et al. (1985) Annals of the New York Academy of Sciences 454, 69-78; Camejo et al. (1990) European Heart Journal 11 (Suppl. E), 164-173; Camejo et al. (1993) Journal of Biological Chemistry 268, 14131-14137; Pentikäinen et al. (1997) Journal of Biological Chemistry 272, 7633-7638.; Steele et al. (1987) Atherosclerosis 65, 51-62.; Alavi et al. (1989) American Journal of Pathology 134, 287-294; Hurt-Camejo et al. (1997) Arteriosclerosis, Thrombosis, and Vascular Biology 17, 1011-1017; Camejo et al. (1998) Atherosclerosis 139, 205-222; Goldberg et al. (1998) Journal of Biological Chemistry 273, 35355-35361; Borén et al. (1998) Journal of Clinical Investigation 101, 2658-2664; Völker et al. (1990) European Heart Journal 11(Suppl. E), 29-40; Skålén et al. (2002) Nature 417, 750-754.). For a number of years it has been suggested that variations in the intimal proteoglycan composition could in part explain the marked differences in atherosclerosis susceptibility between different sites in the vasculature (Tracy (2003)).
Lumican is a member of the small-leucine-rich (SLR) family of proteoglycans. Based on sequence homology, this family is divided into three classes (Iozzo (1999) Journal of Biological Chemistry 274, 18843-18846). Class I SLR proteoglycans consist of decorin and biglycan, and contain chondroitin/dermatan sulfate glycosaminoglycan (GAG) side chains. In contrast, class II SLR proteoglycans include lumican, fibromodulin, PRELP, keratocan and osteoadherin and contain keratan sulfate GAG side chains. The Class III SLR proteoglycans consist of epiphycan and mimecan, of which the former may contain chondroitin/dermatan sulfate GAG side chains and the latter may contain keratan sulfate GAG side chains. Lumican was so named after first being discovered to be present at high levels in the cornea, where it plays an important role in maintaining corneal transparency (Blochberger et al. (1992) The Journal of Biological Chemistry 267, 347-352; Funderburgh et al. (1991) Journal of Biological Chemistry 266, 24773-24777; Funderburgh et al. (1993) The Journal of Biological Chemistry 268. 11874-11880). Lumican is in fact the most abundant keratan sulfate proteoglycan in corneal stroma. In the cornea, lumican maintains corneal transparency predominantly through the proper ordering of collagen, but also likely by inhibiting cellular proliferation and thus contributing to the low cellularity and immune-privileged nature of the cornea (Vijayagopal et al. (1996) Atherosclerosis. 127, 195-203.). Targeted deletion of the gene for lumican in mice, results in corneal opacity as well as fragile skin (Saika et al. 2000; Chakravarti et al. (1998) The Journal of Cell Biology 141, 1277-1286; Chakravarti et al. (2000) Investigative Ophthalmology & Visual Science 41, 33656-3373).
Lumican is composed of a 38 kDa core protein to which may be attached to asparagine residues up to 3 keratan sulfate GAG side chains and/or 2 to 3 oligosaccharides not containing keratan sulfate (Dunlevy et al. (1998) The Journal of Biological Chemistry 273, 9615-9621; Nilsson et al. (1983) The Journal of Biological Chemistry 258, 6056-6063; Midura & Hascall (1989) The Journal of Biological Chemistry 264, 1423-1430). The keratan sulfate chains are composed of linear repeating disaccharide units of N-acetylglucsoamine and galactose (N-acetyllactosamine), and are attached to the protein via an N-glycosidic linkage to N-acetylglucsoamine within a mannose-containing linker oligosaccharide (Nilsson et al. (1983); Funderburgh et al. (2000) Glycobiology 10, 951-958.). The core protein is also known to be sulfated on tyrosine residues (Onnerfjord et al. (2004) The Journal of Biological Chemistry 279, 26-33). The isolated core protein, free of carbohydrate modifications (“lumican-P38”), is often observed in cultured cells, but typically not in human tissues (Qin et al. (2001) J. Pathol. 195, 604-608). In tissue, lumican may be present as a 55 kDa low-sulfated glycoprotein (“lumican-P55”), or as a keratan sulfate proteoglycan (“lumican proteoglycan”), with an apparent mass ranging from 60-100 kDa (Sztrolovics et al. (1999) Spine 1765-1771; Funderburgh et al. (1991); Dolhnikoff et al. (1998) Am J. Respir. Cell Mol. Biol. 19, 582-587; Qin et al. (2001)). Lumican-P55 appears to be relatively widely expressed, being present in most normal tissues, including arterial adventitia (Funderburgh et al. (1991)). In contrast, lumican proteoglycan appears to have a more restricted distribution (e.g., present primarily in normal cornea and cartilage) and have biological effects that are distinct from those of lumican-P55.
There is evidence that some lumicans may play a role in atherosclerosis. One study showed that a keratan sulfate proteoglycan was upregulated in arterial lesions of cholesterol-fed pigeons, compared with normal arteries (Robbins et al. (1992) Arteriosclerosis and thrombosis 12, 83-91). A large-scale gene expression profiling revealed lumican to be one of 55 gene-products upregulated in human atherosclerotic coronary arteries compared with normal coronary arteries (Archacki et al. (2003) Physiology Genomics 15, 64-74). The lumican gene is also expressed in the aortas of mice fed a high-fat diet (Tabibiazar et al. (2005) Arteriosclerosis Thrombosis and Vascular Biology 25, 302-308). By immunohistochemistry, in normal human coronary arteries, lumican is present in the adventitia but absent from the intima (Onda et al. (2002) Expirimental and Molecular Biology 72, 142-149). This adventitial lumican represents the widely expressed lumican-P55, which has been purified from normal bovine aorta (Funderburgh et al. (1991)). However, human intimal hyperplasia and atherosclerotic lesions both demonstrate immunoreactivity for lumican within the intima (Onda et al. “(2002)).
A further understanding of the proteoglycan composition in the thickened intima may lead to the development of therapeutics and diagnostics that target the progression of intimal hyperplasia to atherosclerosis.